Your code has variables that can be (accidently!) used outside of the necessary scope. This is dangerous.

Yes, but it's good to realize that "use strict" doesn't magically fix that. Accidently using variables outside of the necessary scope happens when variables are defined using a scope that's too broad. It's pretty hard to accidently use variables from a different package - you would have to explicitely point to that package. Problems are cases like: